Prokaryotic Flagellum

Bacterial flagella are long, helical, whip-like appendages that give many bacteria their motility, and they are considerably thinner than the flagella or cilia found in eukaryotic cells. The long, exposed filament of a bacterial flagellum is built from a single structural protein called flagellin. Moving inward from the tip, the flagellum's structure consists of the long helical filament, then a short curved hook, and finally a basal body, which is physically anchored into both the bacterial cell wall and the cytoplasmic (plasma) membrane. This basal body is not a passive anchor - it carries a set of rings that function together as a genuine rotary motor, and it is the rotation of this basal-body motor that spins the whole helical flagellum and propels the bacterium forward. The exact ring arrangement differs depending on the bacterium's cell-wall type. Gram-positive bacteria have a comparatively simple basal body with just two rings: an S-ring, attached to the inner face of the peptidoglycan layer, and an M-ring, attached directly to the cell membrane. Gram-negative bacteria, whose cell envelope is more complex, have four rings arranged as two connected pairs: an outer pair - the L-ring (anchored in the lipopolysaccharide layer) and the P-ring (anchored in the peptidoglycan layer) - and an inner pair - the S-ring and M-ring (anchored at the cell membrane) - with all four rings linked together by a central rod running through the middle. Critically, the energy that actually powers this rotary motor is not ATP at all; instead, the rotation runs entirely on a flow of protons. This is described as the proton motive force - the combined electrical potential and hydrogen-ion (proton) concentration gradient that exists across the bacterial cytoplasmic membrane. As protons flow back into the cell through the rings of the basal body, that flow physically driv …